The present invention relates to screen changing apparatus for use with extruders of plastics, rubber and other material. The screen changer supports a strainer plate having a strainer or screen pack through which the extruded material passes for removing impurities and providing the necessary backpressure in the extruder cylinder. After a period of operation, the strainer plate screen becomes clogged with impurities and unplastified material and must be changed or replaced with a clean screen. An example of a screen changer of this type is shown and described in my U.S. Pat. No. 3,675,934 dated July 11, 1972.
One of the problems experienced with screen changers is that when the replacement strainer plate is substituted for the strainer plate having the clogged or dirty screen, the air in the replacement screen results in air bubbles or discontinuities in the extruded product. Some screen changers have ports in the seal plates with a valve for opening the ports to the extruder passage for communication of the extruded material from the extruder passage to the screen of the replacement strainer plate. A bleed valve in communication with the screen of the replacement plate vents the air from the strainer. This arrangement provides prefilling and venting of the strainer of the replacement strainer plate; however, the ports must be kept open and free of contamination to be effective. The valves must be maintained and operated at the proper times.